Temp gauge show Overheating but engine is cool.

Hi,
Can somebody give me advise. My nissan micra car show that engine has overheated(The temp gauge jumps to HI) though the engine appears to be cool. The gaure returns to normal temperature after I turn off the ignition and start again. But again goes up after some running of the engine. Mechanic says that there may be an problem. Please advise.
CVP. If you are looking for a "good guess" I would try anew temperature sending unit and see what happens. It is a cheap guess and probably the most likely. The temp sender is a small sensor that screws into the engine and measures the temp of the coolant then relays the info to the temp gauge on your dash. This is NOT the same thing as a coolant temperaure sensor, that tells the computer how hot the coolant is!!

Keep us posted. Hi Austin,
I will check with the garage and fix the problem. I have run the car with the temp gauge at high for some time. I have noticed that the radiator fan turns on automatically to cool the coolant and shuts off by itself after a minute or so. I also opened the radiator cap ( Using cloth cover) with the engine running and temperature at hi on gauge. The coolant doesn't appears to be too hot.
Thanks for the advise. I did not know about the temp sending unit before.
Regards,
CVP CVP wrote:I also opened the radiator cap ( Using cloth cover) with the engine running and temperature at hi on gauge. The coolant doesn't appears to be too hot.


Respectfully.... what the hell are you thinking! I realize there is a safe way to determine whether it's 'safe' to open the radiator cap (like squeezing the radiator hose to see if there is any pressure in the system), but if you suspect an overheating engine, you should not open the cap.

Overheating coolant is over 250 degrees F! A towel will just confine the burns to the palm of your hand if it spews coolant at that temp. I learned my lesson once by having coolant shoot right up the sleeve of my coat (like, to my armpit). I was expecting searing pain. Luckily the coolant was not that hot. If your car was overheating, the release of the pressure would have allowed the coolant to boil, giving the coolant a geyser effect. A geyser of pain and misfortune.

You can feel the air that is coming out of the radiator fan. You can smell if the engine is overheating. If the fan is kicking on and back off, then you can be sure you are not overheating. The fan would not shut off once it kicks in unless the engine has cooled enough, or the electricity is cut off to the fan because of a fuse (etc).

Be careful and good luck. I’d bet the sending unit is your trouble. Hi eyknough,
Thanks for the advise. Anyway, I changed the Temp sending unit today. Still the problem recurred. I wonder if the problem is with the gauge. When I turn off the ignition for 2-3 minutes and turn on, the gauge returns from all the way high position to the usual midway range. After running the engine for sometime, the gauge goes up all the way suddenly. Again the fan works fine: Starts on and off automatically. Can you give me anymore suggestions?
CVP. If you are not concerned with keeping everything factory, an aftermarket temperature gauge is not difficult to install.

If it were my problem, an aftermarket gauge is what I'd choose. They're pretty accurate and long-lived.

I doubt the problem is with the gauge it's self. Probably more likely that it's the wires running between the sending unit and the gauge. Not too fun to diagnose.

For the heck of it, you could have a friend look at the gauge with the key on (engine off). Ground the wire that hooks up to the sending unit. It should peg the gauge. Speaking of which... when the gauge 'pegs' it's self, it is very likely that the wire hooked to the sending unit has a short to ground somewhere.

The sending unit is basically like a light dimmer (a rheostat). When the sending unit it cold, it conducts no (or very little) current to ground through the wire hooked to the sending unit. As the engine (and sending unit) warms up, it begins conducting gradually more current. The hotter it gets, the more current it allows. If there is a 0 resistance path to ground for the wire, it max's out the gauge on hot. The gauge basically reads how much current the sending unit is allowing to go through it, and reads accordingly.

An aftermarket gauge will be complete with a temp probe that screws right in to where the current temp sending unit goes. The probe is connected to the back of the aftermarket gauge, and the gauge reads how hot the probe is. No electricity required. (Except for the light in the gauge, of course)

Sorry about the lecture on hot coolant... I'm sure that's not what you came here for. Just remember for future reference, please! Car Repair Talk's forum.